Cannot pickle lambdas from SymPy

[parnmatt]

I have been using SymPy recently. Anonymous functions can be generated using sympy.lambdify.
I required to store these for later, as they can be computationally expensive to generate the expressions in the first place; i.e., solving an ODE system symbolically.

dill came to mind as I have previously used it to pickle a lambda, however it was giving errors. Here is a minimalistic example of a simple function:

import sympy
import dill

# use basic session, puts multiple symbols in scope
sympy.init_session()

# create expression, simple for example
expr = 2*x + y

# generate anonymous function
func = sympy.lambdify((x, y), expr)

print(func(5, 6))  #=> prints 16

# pickle file
pickled_func = dill.dumps(func)

Where I get a big error that traces back to:

PicklingError: Can't pickle : it's not found as mpmath.ctx_mp_python.constant

Now I thought this maybe an issue with how SymPy generates the function, however cloudpickle was able to successfully pickle the function:

import pickle
import cloudpickle

print(pickle.loads(cloudpickle.dumps(func))(5, 6))  #=> prints 16

It was brought to my attention that this may be unexpected behaviour with how dill interprets lambdas of objects; and thus you should be made aware.

[mmckerns]

If you use the flag recurse=True on dumps, dill can pickle your func (see #86). This behavior will likely become the default setting once #105 is resolved.

[mmckerns]

added global setting dill.settings['recurse']

[mmckerns]

@mvanvleet: would you mind deleting your comment and posting it as a new issue? You can always refer back to this issue with a link.